Circulation Regimes

The airlift bioreactor and bubble column have very similar bubble-bubble interactions and behavior, which leads to almost identical gas flow regimes and progression. These have been covered in detail in Section 7.2 however, more attention is placed on liquid flow behavior in airlift bioreactors since the liquid phase is a significant source of momentum and gas recirculation. 

The process of gas entrainment and circulation is complicated and not easily quantified. Problems arise from an abstract relationship between the liquid and gas phases. On the one hand, the gas flow rate affects the liquid flow rate through the gas holdup and hydraulic pressure differential relationship. As the gas flow rate increases, larger bubbles rise faster and increase the circulation velocity. A higher circulation velocity, in turn, would decrease the slip velocity and make entrainment easier. On the other hand, if the liquid velocity is higher than the bubble rise velocity, bubbles would experience a drag (lift) force, which would aid entrainment. 

A simple measure to correlate liquid circulation velocity with the superficial gas 

A more detailed correlation conld be arrived at by accounting for each force. For example, one would balance the hydrostatic pressure difference with bioreactor-specific pressure drops (e.g., head losses due to wall fiiction, elbows, and bends). This approach would be based on a theoretical foundation, but attempts have not been very successful due to the specific geometric dependence, and empirical correlations are still the most practical approach (Albijanic et al., 2007 Chisti, 1989). 

Analysis could be skewed to favor riser versus downcomer data due to the riser cross-sectional area usually being larger than that of the downcomer. A better strategy would be to analyze each section separately. One approach is to assume a large degree of independence, which would work relatively well if the controlling factors in each section are independent of the other. A large effective riser diameter would 

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Proshutinsky, A. and Johnson, M. (1997) Two circulation regimes of the wind-driven Arctic Ocean. Journal of Geophysical Research, 102, 12493-12512. 

The paleoclimatic expression by clay mineral successions is direct or indirect, i.e., it either indicates the climate that actually prevailed at a given period, or reflects other events depending on climate migration of lithospheric plates across successive climatic zones, varying extension of ice caps controlling the surficial erosion, variations in the marine circulation regime due to changing latitudinal and... 

Figure 8.4 Circulation regime progression in a draught tube internal-loop airlift bioreactor (van Benthum et al., 1999b), where is the downcomer liquid velocity and is the gas slip velocity.

Once the gas is in the downcomer, the liquid has to flow even faster to cause circulation. Gas bubbles are stiU lighter than the liquid and have a buoyant force, which propels them to rise against the flow. The liquid-phase momentum has to provide the power to overcome the buoyant force and create a net downward force in order to cause forward motion and eventual circulation. In effect, a superficial liquid velocity exists at which gas bubbles cau be suspended or are stagnant in the downcomer (regime 2 in Figure 8.4). Hence, this circulation regime is referred to as the transition regime. 

Wei, Y, Tiefeng, W, Malin, L., and Zhanwen, W. (2008), Bubble circulation regimes in a multi-stage internal-loop airlift reactor, Chemical Engineering Journal, 142(3) 301-308. 

The particle movement is directed upwards inside the tube and downwards in the annulus as it can be seen in Fig. 20. This indicates that the circulating regime is intact over a wide fluidization range. From a height of... 

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